Wireless communication networks are well known and constantly evolving. For example, universal mobile telecommunications system (UMTS) is one of the third-generation (3G) cell phone technologies. Currently, the most common form of UMTS uses wideband code division multiple access (W-CDMA) as the underlying air interface, as standardized by the third generation partnership project (3GPP).
Currently, UMTS networks worldwide are being upgraded to increase data rate and capacity for downlink packet data. In order to ensure a further competitiveness of UMTS, various concepts for UMTS long term evolution (LTE) have been investigated to achieve a high-data-rate, low-latency and packet optimized radio access technology.
3GPP LTE (long term evolution) is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future requirements. Goals of the project include improving efficiency, lowering costs, improving services, making use of new spectrum opportunities, and better integration with other open standards. The LTE project is not a standard, but it will result in the new evolved release 8 of the UMTS standard, including mostly or wholly extensions and modifications of the UMTS system.
A characteristic of so-called “4G” networks including Evolved UMTS is that they are fundamentally based upon transmission control protocol/internet protocol (TCP/IP), the core protocol of the Internet, with built-on higher level services such as voice, video, and messaging.
A sounding reference signal (SRS) may be typically transmitted with a wide bandwidth for a node B (i.e., a base station) to find the best resource unit (RU) for a transmitting from a user equipment (UE). However, due to the restrictions on the maximum UE transmission power, the channel quality indication (CQI) measurement accuracy may be degraded when the SRS signal is degraded, such as when a UE located near edge of the cell transmits the SRS. This degradation of the SRS may cause errors to arise in the optimum RU assignment and in the modulation and coding scheme (MCS) selection. Therefore, improvements in the transmission of the SRS from the UE helps to achieve the maximum user throughput. Accordingly, the SRS may be designed to enable channel aware scheduling and fast link adaptation for PUSCH for UL data transmissions. The SRS is also used as a reference signal (RS) for closed loop power control (PC) for both physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH).
In the current LTE, aspects of the SRS are semi-statically configurable by the UE, for example as part of a radio resource control (RRC) signaling. In particular, the UE may specify various attributes as part of an uplink communications to the node B. For example, changes in the SRS may be used to modify the bandwidth (BW) used by the UE, such as to request for either a narrowband or a wideband SRS BW for a given operating bandwidth. When adjusting the bandwidth, the SRS transmission ideally should not puncture the PUCCH region, which may also occur with a persistent PUSCH.
The UE may also adjust the duration of the SRS. For example, the SRS may be defined as being either “one shot” transmissions or indefinite transmissions that is valid until otherwise disabled or until the session ends. The UE may further adjust the period for the SRS. For example, the period may be 2, 5, 10, 20, 40, 80, 160, or X ms. The UE may further adjust the SRS to include a cyclic shift of 3 bits, as described in greater detail below.
Also, it has been decided that a cyclic shift of the SRS sequence is indicated by 3-bits. It may be possible to indicate 23, or 8, different cyclic shift values using the 3-bits. However, the question that arises is how to maximize the cyclic shift separation between the SRS resources.
Another problem that arises due to the above-described UE-based customization of the SRS is supporting code-tree based bandwidth assignment with maximized cyclic shift separation.
To provide an efficient assignment of SRSs with different transmission bandwidths, one conventional scheme presents a bandwidth assignment based on orthogonal variable spreading factor (OVSF) code assignment with a tree structure. Although the present discussion refers to OVSF, it should be appreciated that other tree-based assignments, such as Walsh codes are known and may used in the alternative.
OVSF and other tree-based codes may support both hopping-based and localized-based multiplexing for SRSs with a narrower transmission bandwidth than the system bandwidth in order to maximize the user throughput performance in various cell deployment scenarios. Moreover, the conventional scheme may be adapted to achieve an efficient SRS hopping method based on the switching of branches of the OVSF code tree. However, this conventional scheme does not take into account the current SRS assumptions made in 3GPP. For example, the scheme may not work properly if the SRS transmission puncture the PUCCH region or if certain BW options are allowed for SRS.